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| Book/Report | FZJ-2018-04469 |
1993
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19463
Report No.: Juel-2810
Abstract: Nonlinear optical spectroscopies using second-harmonic and sum-frequency generation are powerful techniques to examine properties of surfaces and interfaces.However, they have been applied only rarely so far, mainly because appropriate highpower laser systems with large and easy tunability are not commonly available. This work discusses the development of such a tunable laser source as well as the experimental setup for nonlinear-optical spectroscopy of surfaces and interfaces which is applied to the investigation of electronic and vibrational properties of silicon surfaces and interfaces. The technique of optical parametric generation and amplification is utilized to achieve tunability. Intense radiation in the visible and near infrared is obtained from an optical parametric generator and amplifier with two lithium borate (LiB$_{3}$O$_{5}$) crystals pumped by the third harmonic of a picosecond Nd:YAG laser. A diffraction grating between the two crystals spectrally narrows the generated radiation. A subsequent optical parametric amplifier consisting of two silver gallium sulfide (AgGaS$_{2}$) crystals is used to extend thespectral range to the medium infrared. The prominent feature of the system is the continous tunability of the wavelength from 0.41 $\mu$m (violet) to 12.9 pm (medium infrared). Such a wavelength tunability by a factor of 30 has not been achieved by other laser systems so far. High conversion efficiencies and output pulse energies are obtained: the power of the pulses reaches up to 60 MW, with pulse durations of 15 to 19 ps. The advantage of this tunable high-power laser system over commonly used picosecond dye lasers is the much larger continous tunability. Also, the output energy varies weaker with the wavelength than for dye lasers.Quick wavelength tuning is achieved by rotating the crystals and the grating. Due to these favorable properties, the laser system is a very convenient source for nonlinearoptical interface spectroscopy by second-harmonic generation (SHG) and sum-frequency generation (SFG). The large tunability allows the measurement of sum-frequency spectra with the excitation wavelength varying more than one order of magnitude. The theoretical background of sum-frequenq generation at interfaces is discussed. For the first time, an analytical relation between the sum-frequency intensity and the tensor components of nonlinear interface susceptibility and nonlocal bulk susceptibility is explicitly given for (100) and (111) interfaces of cubic media. Surface and interface properties of clean, oxidized and hydrogen-terminated silicon samples are investigated by SHG and SFG spectroscopy. Spectra of Si-SiO$_{2}$ interfaces exhibit a resonance band at 3.3 eV photon energy. Appearance and intensity of the band strongly depend on post-oxidation treatment, but not on oxide thickness. Likewise, the band is observed for clean Si(100)2x1 and Si(111)7x7 surfaces whereas it is absent on the wet-chemically prepared, hydrogen-terminated Si(100)1 x 1 and Si(111)1 x 1 surfaces. SHG measurements during oxygen adsorption on the clean Si(111)7x7 surface rule out Si dangling bonds to be responsible for the resonant enhancement. Comparison to the resonance of the linear bulk suceptiblity of silicon at 3.4 eV, caused by direct E$_{1}$ interband transitions, allows to relate the resonance in the nonlinear susceptibility to direct interband transitions in a thin layer of silicon with vertically strained Si-Si bonds at the selvedge of the clean surfaces and at the Si-SiO$_{2}$ interface. This interpretation is consistent with indications of distorted layers at the Si-SiO$_{2}$ interface, obtained from ion backscattering channeling data. It is also in agreement with models of the Si-SiO$_{2}$ interface that assume strained Si-Si-bonds to account for the U-shaped density of interfaces states commonly found in capacitance-voltage measurements. Sum-frequency measurements of clean silicon surfaces reveal additional bands at 0.57 eV on the Si(100)2x1 surface and at 0.4 eV on the Si(111)7x7 surface. They appear upon adsorption of various molecules. These bands are therefore assigned to transitions incorporating states of silicon dangling bonds at the surface. Further experiments explore the potential of sum-frequency generation for vibrational spectroscopy of adsorbate-covered surfaces and demonstrate the versatility of the laser system. A number of adsorbate and adsorbate-substrate vibrations on clean silicon surfaces are detected. The dissociative adsorption of water on the Si(100)2x 1 surface and of ammonia on the Si(111)7x7 surface, previously investigated by electron energy loss spectroscopy (EELS) and infrared reflection absorption spectroscopy (TRAS), can also be observed by the nonlinear-optical technique. Also, the structural phase transition of the Si(100) surface from the 2x1-reconstructed monohydride phase to the unreconstructed dihydride phase upon adsorption of atomic hydrogen is observed with optical sum-frequency generation.
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